Retractable safety gates composed of flexible barrier sheets and winding mechanisms are known. One example is disclosed in PCT patent publication WO94/00664, in which a roll of flexible sheet material is disposed in a housing that is vertically mounted to one side of a passageway. When the sheet material is extended across the passageway, it is received and releasably held by another housing mounted to the opposite side of the passageway. To accommodate passageways of varying widths, the barrier sheet will often be longer than the passageway it extends across. Consequently, a lock mechanism must be provided to prevent over-extension of the barrier once in position in the passageway. PCT patent publication WO94/00664 discloses a lock mechanism that is composed of a plate and a key pin that is introducible into a selected one of a plurality of locking holes in the plate. U.S. Pat. No. 7,219,709 discloses a retractable safety gate having a spring-loaded ratchet and pawl mechanism to lock the barrier in the extended position. U.S. Pat. No. 7,178,792 discloses adjustable width safety gate having an electrically operable locking mechanism. U.S. Pat. No. 6,907,914 discloses a rotating catch wheel that cooperates with a pinned driver locking and unlocking mechanism for a safety gate. U.S. Pat. No. 6,435,250 discloses a retractable safety gate having a clutch and stop mounted on a spool for locking the barrier in its extended position. U.S. Pat. No. 5,690,317 discloses a retractable safety gate having a spring mounted roller mechanism that is automatically locked against rewinding of the barrier by means of a ratchet and pawl mechanism. United States Patent Publication No. 2011/088323 discloses a retractable safety gate locking mechanism that is composed of a release wheel including a cylinder and a cam, a spring-loaded lock actuator movable by rotation of the cam, a toothed member that engages and disengages with the lock actuator, and a timer operatively connected to the release wheel.